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What is Midbrain?

MVSAT 2024

Midbrain or mesencephalon is located at the base of the brain connecting two other major brain parts; forebrain and hindbrain. Midbrain is the most rostral part of the brainstem, i.e., it is located at the tip of the frontal lobe, but also the shortest part of the brainstem. It formed the topmost part of the brainstem and is located below the cerebral cortex. Midbrain contains many important structures connecting the spinal cord to the brain. This tiny but mighty structure plays a crucial role in dealing with hearing, vision, movement, pain, arousal, and sleep.

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Midbrain Anatomy 

The midbrain is the topmost part of the brainstem, which connects the brain with the cervical spinal cord. The Midbrain parts consist of three parts, namely, the colliculi, the tegmentum, and the cerebral peduncles.

The colliculi are the topmost midbrain parts originating from the Latin word meaning hill. It has two pairs of stuffed, encrusted bundles of neurons known as superior and inferior colliculi. The superior colliculi process the visual signals before they reach the occipital lobe situated at the back of the skull. The auditory signals are processed by inferior colliculi before they pass through the thalamus, followed by the chief auditory processing centre located at the cortex. The two neurons form a bump on the posterior external surface of the brainstem.

In the anterior midbrain parts, the tegmentum elongates down the brainstem, but a portion forms a part of the brainstem. The tegmentum midbrain structure contains two areas the periaqueductal gray matter (PAG) involved in controlling pain, the iron-enriched red nucleus, which appears pink responsible for movement coordination, sensory and motor nerve pathways, certain cranial nerve nuclei, and substantia nigra.

A pair of large nerve fibre bundles transverse the back of the midbrain connecting the rest of the brainstem to the forebrain. The cerebral peduncle acts as a pathway to transfer signals from the cortex to other parts of the central nervous system. Between two midbrain sections, cerebral peduncle and tegmentum, lie a layer of dark pigmented (melanin) cluster of neurons with cells known as substantia nigra. This layer is a vital transmitting station for nerve signals of the central nervous system as it produces neurotransmitter dopamine. This layer also controls body movement. In Parkinson's disease, this area gets affected.

Midbrain Blood Supply 

The midbrain anatomy is stuck between the thalamus and pons, measuring around 1.5 centimetres; the lateral side of the midbrain is enclosed and hidden by the hippocampal gyri of the brain, as shown in the midbrain diagram inserted above. The cerebellar artery (Superior) supplies blood to the tectum of the midbrain. The central part (tegmentum) receives blood from the posterior cerebral artery and paramedian branches of the basilar artery.

Location and Function

Midbrain location is at the base of the skull; though it is the smallest area of the brain, it is an important processing centre for visual and auditory signals. Midbrain function involves free movement of body and head, as it provides passage for downward pathways for the cerebral cortex. It is also a channel for the chief rising spinal cord that transmits stimuli (sensory) from the head and body to the direct brain. The mesencephalic part of the reticular portion, along with other brainstem portions, control breathing, pain, and mood swing. Midbrain function psychology invokes survival instincts just by identifying potential hazardous objects and behaviour.

Substania nigra situated in the midbrain has the extrapyramidal motor nerve, thus controlling voluntary movements. Midbrain also facilitates eye movements and ocular and auditory reflexive movements. The substantia nigra plays a role in reward, addiction, and movement due to the high level of dopaminergic neurons. In Parkinson's disease, the level of dopamine reduces substantially, degeneration of substantia nigra is obvious.  

The midbrain is a small structure of the brainstem, one of the three parts. Though it is a small cluster of neural tissues, it consists of many nuclei, nerves, pathways, and other structures, each with discrete functions. The spectrum of action of the midbrain is wide, and it is difficult to explain in a few paragraphs. One of the most distinctive external features of the midbrain is the existence of four bumps on the posterior surface. Under those bumps, there lie four collections of neurons, the upper pair of super colliculi is responsible for behavioural response, and the lower pair of inferior colliculi is in charge of auditory processing. 

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FAQs on Midbrain

1. What is the importance of the brainstem?

In vertebrate anatomy, the brainstem structurally joins and continues with the brain and spinal cord. Cranial nerves originate from the brainstem, which provides main motor and sensory innervations to the face and neck. Though it is a small structure, a vital portion of the brain as the motor and sensory from the brain passes through the brainstem, connecting with the peripheral nervous system. The motor and sensory nerves give rise to fine touch, vibration sensitivity, and proprioception, and the spinothalamic tract controls pain, temperature, itch, and crude touch. The brain stem also regulates cardiac and respiratory functions. Additionally, the brainstem regulates the central nervous system (CNS) and is critical to maintaining consciousness and sleep cycle.

2. What is periaqueductal gray (PAG)?

The periaqueductal gray (PAG) is a region of gray matter found in the midbrain. The PAG environs the cerebral aqueduct and forms a column in the brainstem, measuring around 14 millimetres in length. Although there is no visible division in PAG, researchers have divided it into four columns according to their connectivity and functions; the dorsomedial, dorsolateral, lateral, and ventrolateral columns. Although the functions of FAG are multifaceted and not fully comprehended, its association with the sensation of pain is established. It plays an important role in pain transmission; it transmits the pain signal to the cortex. FAG is recognized as a vital region for pain inhibition.

3. What causes Parkinson's disease?

Parkinson's disease is a nervous system disorder hampering body movements. The symptoms of Parkinson's disease are muscle stiffness, trembling, trouble in maintaining balance, coordination, and walking. With time the symptoms deteriorate. This disease occurs when substantia nigra in the midbrain gets impaired or degenerates. Substania nigra produces the neurotransmitter hormone dopamine that helps in transmitting signals between brain areas. When these nerve cells get impaired production of dopamine ceases or lessens. Dopamine is vital for the functionality of basal ganglia, the area of the brain responsible for systematizing brain command of body movements. The level of another neurotransmitter called norepinephrine also decreases, which is essential to maintain automatic functions such as digestion, heartbeat, blood pressure, and breathing.